I have also considered this possibility. The good part is that you could build a fire and it would burn efficiently a long time getting the water up to temperature - thereby having a nice long time to burn efficiently and as smokeless as possible. If you could size the tank so that it could keep you heated for 12-24 hours - You would only have to build a fire once or twice a day and it would burn completely down to coals if you were a good judge at how much wood to add. The tank could either be built next to the OWB or placed in the house - and it could easily be insulated to store the heat.
The bad news is that if you put too much wood in - the OWB would be in the smolder mode for a long, long time and when it finally fired up it could smoke like crazy for even longer than normal. Also the OWB controls may not work real well with the system. In the case of my Woodmaster the blower shuts off when the water reaches 170 degrees, then comes back on at 150 degrees - and the OWB blower shuts down at 120 degrees as it senses the fire is out. This system works just fine as I put in enough wood to keep the water from dropping below 150 most of the time. With the expanded water supply if I put in what I thought was an adequate amount of wood and I got the temperature of the water up to 170 just as the wood supply ran out - then if the temperature dropped below 150 and the fan came on - it would continue to run the fan non-stop and blow cold air inside the furnace until the temperature dropped below 120. (This would waste lots of energy when the blower was moving cold air through the inside of my furnace with no wood inside to burn). Even on the natural draft furnaces this could be an efficiency killer as the outside air would come inside the OWB and get heated up and go out the chimney as long as the OWB sensors were trying to keep a non-existant fire going. I suppose you could install some form of controls so that the blower (damper) would not come on after the first cycle.
Unless you are going to use the extra heat capacity to allow you to only build one or two fires a day that burn completely - I don't really see any benefit in the extra water capacity. As the weather gets warmer I may see if it is possible for me to keep my domestic hot water heated by only burning one clean/efficient small fire a day. I may have a problem with this on wet rainy days as my furnace does not have a chimney cap and rainwater can enter my furnace - the fellow I bought the furnace from did not recommend a cap as the creosote builds up on the cap and drips on the furnace and makes it look bad.
